March 22 (UPI) -- Alabama Gov. Kay Ivey signed a bill Thursday to allow executions by nitrogen hypoxia as an alternative to lethal injection.

Alabama became the third state after Oklahoma and Mississippi to allow executions by nitrogen hypoxia, which kills by cutting off oxygen to the brain and increasing nitrogen until the victim stops breathing.

Although it's not certain that nitrogen hypoxia causes a painless death, Alabama state Rep. Jim Hill, who supported the bill, said its use in assisted suicide proves it's a safer alternative than other methods of executions, the Montgomery Advertiser reported.

The bill had strong support from the Alabama legislature, passing the Senate 29-0 in February and the House 75-23 on Tuesday.

As drugs for lethal injections become more difficult for states to obtain, nitrogen hypoxia has emerged as an alternative. In February, Oklahoma decided to use the nitrogen hypoxia as its primary method of execution after two problematic lethal injections.

"Using an inert gas will be effective, simple to administer, easy to obtain and requires no complex medical procedures," Oklahoma Attorney General Mike Hunter said last month.

But some oppose the use of nitrogen hypoxia in executions.

"This method has never been used before and is experimental," said Dale Baich, an attorney for Oklahoma death-row prisoners challenging the new nitrogen protocol.